1. Field of the Invention
The present invention relates in general to waveguide circulators for the non-reciprocal transmission of microwave energy; and more particularly to a novel system for reducing the size, mass, and insertion loss of the transition between adjacent circulators.
2. Description of the Related Art
Multi-junction waveguide ferrite circulator assemblies have a wide variety of uses in commercial and military, space and terrestrial, and low and high power applications. A waveguide circulator assembly may be implemented in a variety of applications, including but not limited to LNA redundancy switches, T/R modules, switch matrices, and switched-beam antenna systems. Ferrite circulators are desirable for these applications due to their high reliability, as there are no moving parts required. This is a significant advantage over mechanical switching devices. In most of the applications for multi-junction waveguide switching and non-switching circulators, small size, low mass, and low insertion loss are significant qualities, for example, in switched-beam antenna arrays where switches are located directly behind an antenna array.
A commonly used type of waveguide circulator has three waveguide arms arranged at 120° and meeting in a common junction. This common junction is loaded with a non-reciprocal material such as ferrite. When a magnetizing field is created in this ferrite element, a gyromagnetic effect is created that can be used for circulating the microwave signal from one waveguide arm to another. By reversing the direction of the magnetizing field, the direction of circulation between the waveguide arms is reversed. Thus, a switching circulator is functionally equivalent to a fixed-bias circulator but has a selectable direction of circulation. Radio frequency (RF) energy can be routed with low insertion loss from one waveguide arm to either of the two output arms. If one of the waveguide arms is terminated in a matched load, then the circulator acts as an isolator, with high loss in one direction of propagation and low loss in the other direction.
For applications where additional isolation is required between waveguide ports or where additional input/output ports are required, multiple waveguide circulators and isolators are used. The most basic building blocks for multi-junction waveguide circulator networks are single circulator junctions, optimized for an impedance match to an air-filled waveguide interface. For the purposes of this description, the terms “air-filled,” “empty,” “vacuum-filled,” or “unloaded” may be used interchangeably to describe a waveguide structure. The circulators can be connected in various configurations as required for the desired isolation and input/output port configuration. The direction of circulation may either be fixed or switchable.
Conventional waveguide networks comprised of multiple ferrite elements typically have impedance-matching transition and an air-filled waveguide section between the ferrite elements. For example, conventional waveguide circulators may transition from one ferrite element to a dielectric-filled waveguide such as a quarter-wave dielectric transformer structure, to an air-filled waveguide section, and then back to another dielectric-filled waveguide section and the next ferrite element. The dielectric transformers are typically used to match the lower impedance of the ferrite element to that of the air-filled waveguide. The air-filled waveguide section between quarter-wave dielectric transformers is designed to be sufficiently long, generally at least a quarter-wavelength, so as to allow the fields to transition back to the standard waveguide TE1,0 mode between circulators. Thus, the conventional transition between ferrite elements occurs over a length of three-quarters of a wavelength or greater between adjacent ferrite elements. This sets the minimum separation distance that can be obtained in multi-junction assemblies when the input/output ports of multiple circulators are intercoupled to provide a more complex microwave switching or isolation arrangement. This can result in a multi-junction waveguide structure that is undesirably large and heavy. Furthermore, the insertion loss of a multiple circulator assembly increases as the separation distance between ferrite elements is increased as a result of the finite conductivity of the waveguide structure.
Referring now to FIG. 1, there is shown a top view of a conventional two-junction waveguide circulator structure. Magnetizing windings 109 are inserted through apertures in each leg of the ferrite elements 102 in order to establish a magnetizing field in each ferrite element. The polarity of this field can be switched back-and-forth by the application of current on the magnetizing winding to create a switchable circulator. Each of the ferrite elements have a quarter-wave dielectric transformer 110 or 111 attached to each leg. There are two transformers 110 attached to the adjacent legs of the ferrite elements and four transformers 111 attached to the remaining legs of the elements. As shown in FIG. 1, there is a substantial air gap of distance “G” between the quarter-wave dielectric transformers 110 that are attached to the adjacent legs of the ferrite elements. This distance “G” is typically longer than a quarter-wavelength. Dielectric spacers 112 are disposed on the top and bottom surfaces of the two ferrite elements. In FIG. 1, the bottom spacers are hidden from view. These dielectric spacers are used to properly position the ferrite elements in the conductive waveguide structure 100 and to provide a thermal path out of the ferrite elements to the conductive waveguide structure for high power applications. The conductive waveguide structure may include waveguide input/output ports 121, 122, 123, and 124. These ports provide interfaces, such as for signal input and output, for example. Empirical matching elements are not shown, but they may be disposed on the surface of the conductive waveguide structure to affect the performance. The matching elements may be capacitive/inductive dielectric or metallic buttons that are used to empirically improve the impedance match over the desired operating frequency band.
Previous patents have described approaches for decreasing the spacing and loss between the ferrite elements by replacing the standard quarter-wave dielectric transformers with a reduced height waveguide transition. This method removes the quarter-wave dielectric transformers, but the reduced height transition is sensitive to dimensional variations, which results in a design that is expensive and difficult to manufacture and assemble. Other patents have described approaches for eliminating the quarter-wave dielectric transformers between ferrite elements. These methods provide size and insertion loss benefits, but this comes at the expense of isolation and frequency bandwidth.
In view of the problems with the conventional waveguide circulator structures disclosed above, there is a need for a multi-junction waveguide circulator structure that provides improvements in the critical areas of size, mass, and insertion loss without sacrificing the manufacturability or isolation performance of the assembly.